Wealth and big windfall - misunderstood or broken?

Large takes like an inheritance or a haul of treasure valued greater than your current income bracket increase your wealth rating by 1d6 per 25% of value over your income.

Taken literally, this would mean that a treasure with value of 125 % of your income would give +d6 wealth, 150 % of your income would give +2d6 wealth, and so on in a linear fashion.

The example under the text does not support this reading, at least explicitly:

Should he sell it, a diamond ring worth 10,000 livres would be worth +4d6 wealth to a lieutenant of musketeers with a income of 1000s of livres.

If someone has an income of 1000 livres, then 10 000 livres is ten times that, meaning 1000 % of the income. This is 900 % over the income, giving 900 % / 25 % in d6 in wealth, i.e. +16d6 wealth.

If, on the other hand, we take the musketeer’s income to be 5000 livres, then the treasure is worth 200 % income, thus giving +4d6 wealth. Maybe this is how the incomes of thousands of livres is supposed to be interpreted?

Linear increase, exponential system

The general issue here is that wealth increases linearly in the case of a large treasure, while the meaning of wealth is superlinear (something like high polynomial, maybe; slower than straightforward exponential growth in any case). This leads to all sorts of strangeness. This is particularly egregious in case of very poor characters.

Suppose someone has an income of 10 livres and they get a treasure of 110 livres. This is a 1100 % their income, or 1000 % over their income, thus giving an increase of 40d6 to their wealth, likely skyrocketing them far over the king and the state in wealth.

Since the results here are so nonsensical, I suppose I am misinterpreting the rule. How is it supposed to be used?